I. Field of the Invention
The present invention relates generally to variable geometry devices employed in turbine engines and, more particularly, to such a device for use in the diffuser passageway between the turbine engine compressor outlet and the combustion chamber.
II. Description of the Prior Art
A conventional turbine engine includes a support housing, a compressor having an outlet rotatably mounted within the support housing and a diffuser passageway which fluidly connects the compressor outlet to a combustion chamber also contained within the support housing. In many previously known turbine engines, the diffuser passageway is generally annular in shape having in its inner end open to the compressor outlet so that the air flow through the diffuser passageway is generally radially outward. In addition, many of the previously known turbine engines include diffuser vanes extending across the diffuser passageway to aerodynamically control the flow of compressed air from the compressor and to the combustion chamber.
Many turbine engine applications require that the turbine engine be operated over a broad range of operating conditions. These different operating conditions in turn entail different air flow and pressure delivery requirements. Moreover, it is desirable to maintain high turbine engine performance, and thus turbine engine efficiency, at all of the operating conditions which, in turn, avoids surge, cavitation and other engine instabilities.
One previously known method of broadening the flow capacity characteristics in the diffuser passageway is to use variable geometry engine components. In the case of centrifugal compressors, the desired variable geometry for the diffuser assembly is typically accomplished by varying the angle of the diffuser vanes in the diffuser passageway.
The previously known pivoted diffuser vanes, however, have not proven wholly satisfactory in use to vary the diffuser geometry. One disadvantage of this method results from the leakage losses which occur in the diffuser assembly around the pivoted diffuser vanes and into the support housing. These clearance losses are further amplified due to the large openings in the diffuser walls which are required to compensate for thermal distortion and thermal expansion.
A still further disadvantage of the use of pivoted diffuser vanes to vary the aerodynamic geometry of the diffuser passageway is that it is difficult to accurately pivot all of the diffuser vanes to the same angle due to mechanical back-lash and mechanical play. Unwanted and undesired turbulences result when the diffuser vanes are positioned at different angles.
Patent related to varying the flow with a movable side wall diffuser are described in the following patents: U.S. Pat. No. 2,739,782 to White; U.S. Pat. No. 2,996,966 to Jassniker; and French Pat. No. 1,208,697--Societe dite: Sulzer Freres.
These inventions relate to varying the exit diffuser flow area alone and unless the inlet flow area is also simultaneously changed, the efficiency, flow range and pressure ratio is adversely affected. In contrast to the above-described prior arrangements, the present invention contemplates improvements by synchronized area changes at both the inlet and exit of the compressor. In the embodiment of the invention the movable exit diffuser wall will also provide for static pressure equalization at the vane ends by allowing flow from the impeller exit to enter an annular chamber receiving the vanes through vane slots. It is known that interconnecting the vane entrances and equalizing the static pressure leads to increases of compressor range as described in the invention of U.S. Pat. No. 4,121,389 to Perrone et al. The reference invention contemplates pressure equalization to closed chambers on both sides of the diffuser by small openings or slots oriented to lie in planes normal to the flow. In contrast to U.S. Pat. No. 4,121,389 which contemplates wall to wall pressure equalization, the present invention has one continuous slot in the chambered wall for each vane and results in an end static pressure equalization from vane pressure side to suction side resulting in a different mechanism to improve surge and flow range performance.